Renewable energy tapped from tidal or wind forces has been demonstrated in several ways. Various hydrofoil/airfoil turbine designs have been designed and deployed as electrical power generators. The majority of these turbines are coupled to electrical generators to produce electricity, and most use either horizontal or vertical axis with rotor blades to tap energy from the flow, that requires relatively high threshold inflow speed. While this arrangement provides the most uniform drive throughout the cycle, and therefore provides stable power generation, the energy density of this arrangement leaves something to be desired, and the volume they occupy is large and unwieldy. It is difficult to arrange rotary blade turbines compactly in either horizontal or vertical configuration, and therefore in a manner that efficiently harvests energy over a broad area. The fluid forces on rotary turbine blade's surface are unevenly distributed because of the uneven velocity distribution across the span of the blades. In addition to efficiency loss, very heavily loaded blade surface in the tip region may cause a further efficiency reduction due to compressibility effect for wind turbine blades and due to cavitation for tidal turbine blades. Cavitation, in addition to being a source of noise and vibration, is known to fracture tidal turbine blades.
In underwater embodiments, high velocity flows tend to be confined to substantially planar features. Uniform flows are therefore provided within regions that are frequently limited in depth, but are frequently expansive in length and width. Furthermore, many bodies of water are required for surface vehicle navigation and so minimum depths of water clearance must be maintained. Oscillating foil turbines occupy less depth and hence provide more water clearance for a given span, and thus requires less water depth, and can be deployed in a greater variety of locations. Deep water operation demands much higher cost in design, fabrication of turbines, and in installation, maintenance and operations. Oscillating foil turbines are friendlier to marine creatures and less prone to catch fishing nets and seaweed. There are therefore several reasons for seeking viable oscillating foil turbine designs.
The idea of oscillating foil turbines has been around for many years. Instead of spinning blades of rotary blade turbines, you oscillate the foil, typically translating the foil in a direction perpendicular to the direction of flow. Most often oscillating foil turbines displace only in heave and pitch directions, although other motions are known such as motions that additionally involve surge displacements. Foils are known to have different amounts of lift depending on the effective angle of attack (pitch), and as a result of an oscillating foil's instantaneous motion with respect to the direction of flow. By controlling the pitch angle to provide the maximum possible positive lift during a first stroke (typically called an “up” stroke, regardless of orientation), and nearly equal negative lift during a second (“down”) stroke, a substantial net power per cycle may be obtained with an oscillating foil.
These generators can be deployed in isolated coastal communities, inland water ways, and on off-shore installations, as well as for extremely large power generation. Such generators have small environmental impact compared with dams, relatively low installation and operating costs, and do not require highly special coastal geology or marine current dynamics, making them very attractive. However they are still not accepted in the power generation industry and the technology remains substantially underdeveloped.
Pulse Tidal (UK) has a website showing two pairs of oscillating hydrofoils on a single submersible deck. The two pairs are separated horizontally, side-by-side facing the flow, and the two pairs of the foils are in tandem (one fore and the other aft). The paired foils are shown operating 180° out of phase, so that while one foil is on a power stroke, the other is on the return stroke, and vice versa. Furthermore, the second pair is 90° out of phase with respect to the first pair, as the foils of the first pair are near the transitions between power and return strokes, and the second pair is in a middle of the power and return strokes. The document does not describe how the foil motion is converted into electricity, although it is clearly shown that the foils are coupled to a pivoting arm structure that has an articulated elbow joint. Consequently the motion of the foils is of a combination of heave, pitch and surge due to the arcurate motion. Surge motion of the foil is the result of the arcurate motion and this motion produces an added inflow velocity during part of the cycle and reduces inflow velocity in equal measure during an opposite part of the cycle. This change in effective flow velocity is generally not preferred as it decreases efficiency of power generation.
Given the separation of the paired foils in the direction of flow, no dynamic wing-in-ground (WIG) effect is produced between the foils in operation. Given the angled elevating structures to which the arm is mounted, and the arcuate course of the foils that provides greatest surge displacements as the foil approaches the ground, no static WIG effect is produced. In operation, these foils generate lift as per normal hydrodynamic forces on a foil.
WO 2009/068850 to Paish teaches a paired oscillating hydrofoil turbine in which foils run in counterphase and bear rotationally symmetrically on a crankshaft. The foils are pivotably mounted to a frame and a pitch control is provided by a linear actuator attached to the pivoting arm. The pushrods coupling the foil to the crankshaft are also coupled to the pivoting arm.
The turbine shown in FIG. 2 of WO 2009/068850 has a lot of frame in the inflow and outflow paths of the turbine, partially occluding the foils and reducing a velocity of the flow. The longer the pushrods, the less pronounced the reduction in velocity of the flow, but the longer the pushrods, the stronger and heavier they need be to reduce losses and the greater the inertia of the turbine.
WO 2009/068850 does not teach or suggest leveraging the WIG effect. At the instant shown in FIG. 2 the foils appear to be somewhat parallel (given the position of the crankpin, one foil is slightly up stream of the other). It will be appreciated that they drive a crankshaft and so only when one foil is mid down stroke, and the other is mid up stroke, the foils will be aligned in the flow. As their directions of motion are opposite, the angles of attack of the foils are therefore pitched in a same direction (one for positive lift, the other for negative lift, but pivoted the other way) when they are closest, so that adjacent sides are one suction and one pressure. This would decrease any WIG effect that might be present at that juncture, if the foils had been close enough to produce a substantial dynamic WIG effect. But in any case the separation of the foils appear to be far greater than 20% of the chord of the foils, and thus would not produce a meaningful WIG effect.
It is known to use WIG effect in propulsion systems for submersible and surface watercraft, as is taught, for example, in Applicant's U.S. Pat. No. 6,877,692. However propulsors have substantially different properties from passive foils that tap energy from steady flows. The foils have low load compared with propulsor foils, and optimization of foils to generate wing-in-ground effect has not been experimented with in power generation. Indeed propulsors would be expected to have very poor properties if used as a turbine for a generator. The turbines known in the art are not designed to leverage the WIG effect. The floor of a body of water can rarely be used as a ground for WIG effect oscillating foil turbines, because substantial clearance to floor has to be maintained for operation safety, and because the inflow velocity is slowest near the large boundary layer on the floor. The power generated generally varies with a cube of the inflow velocity.
There remains a need for higher efficiency, higher power capacity oscillating foil turbines that are suited for deployment in relatively shallow flows.